1. Field of the Invention
The present invention relates generally to the structure of cutting elements for drill bits for subterranean drilling, and more specifically to cutting elements including cutting surfaces of superhard material, the cutting elements being structured via configuration of the superhard material in combination with that of a supporting structure to provide superior stiffness for the superhard material and enhanced heat transfer from the cutting surface to and through the supporting structure.
2. State of the Art
Superhard materials, normally diamond, have been employed in cutting elements for rotary drill bits for decades. For about the past twenty years there has been widespread use of synthetic diamond cutters, specifically in the form of polycrystalline diamond compacts. Polycrystalline diamond compact cutting elements, commonly known as PDC's, have been commercially available for over 20 years. PDC's may be self-supporting or may comprise a substantially planar diamond table bonded during formation to a supporting substrate. A diamond table/substrate cutting element structure is formed by stacking into a cell layers of fine diamond crystals (100 microns or less) and metal catalyst powder, alternating with wafer-like metal substrates of cemented tungsten carbide or other suitable materials. In some cases, the catalyst material may be incorporated in the substrate in addition to or in lieu of using a powder catalyst intermixed with the diamond crystals. A loaded receptacle is subsequently placed in an ultrahigh temperature (typically 1450.degree.-1600.degree. C.) ultrahigh pressure (typically 50-70 kilobar) diamond press, wherein the diamond crystals, stimulated by the catalytic effect of the metal power, bond to each other and to the substrate material. The spaces in the diamond table between the diamond to diamond bonds are filled with residual metal catalyst. A so-called thermally stable PDC product (commonly termed as TSP) may be formed by leaching out the metal in the diamond table. Alternatively, silicon, which possesses a coefficient of thermal expansion similar to that of diamond, may be used to bond diamond particles to produce an Si-bonded TSP. TSP's are capable of enduring higher temperatures (on the order of 1200.degree. C.) without degradation in comparison to normal PDC's, which experience thermal degradation upon exposure to temperatures of about 750.degree.-800.degree. C.
While PDC and TSP cutting elements employed in rotary drag bits for earth boring have achieved major advances in obtainable rate of penetration while drilling and in greatly expanding the types of formations suitable for drilling with diamond bits at economically viable cost, the diamond table/substrate configurations of state of the art planar cutting elements leave something to be desired.
First, bending attributable to the loading of the cutting element by the formation may cause fracture or even delamination of the diamond table from the substrate. It is believed that such degradation of the cutting element is due at least in part to lack of sufficient stiffness of the cutting element so that, when encountering the formation, the diamond table actually flexes due to lack of sufficient rigidity or stiffness. As diamond has an extremely low strain to failure (diamond cannot tolerate large values of absolute strain), only a small amount of flex can initiate fracture. In addition, fracture may also be initiated in the highly stressed carbide substrate when cutting loads are applied to the cutting element. The carbide is stressed in tension during cooling after the previously-described fabrication process, due to the difference in coefficients of thermal expansion between the diamond and the substrate material.
A second limitation of PDC's is due to excessive buildup of heat due to frictional forces generated during the cutting process. While the superhard material of the cutting element table has an extremely high thermal conductivity (on the order of 400 to over 600 watts/meter Kelvin) and the substrate has a relatively high thermal conductivity (on the order of 100 watts/meter Kelvin), the bit body, typically steel or WC matrix, has a far lower thermal conductivity (on the order of 30 watts/meter Kelvin). As the cutting element wears and the point of contact with the formation becomes an ever-wider wear flat, the cutting element is subjected to higher cutting energies and the substrate becomes ever-smaller, limiting and actually reducing the potential rate of heat transfer. The heat buildup causes overheating of the cutting element and accelerated wear of the diamond table and supporting substrate. In "dull" or used bits, such excessive heating is often manifested on the WC substrate behind the diamond table by the phenomenon of "heat checking", which comprises vertically running fractures in a checkerboard pattern.
It has been proposed to enhance the stiffness of superhard cutting elements by providing the superhard table with a linearly-extending portion of enhanced thickness. Such a configuration provides additional stiffness for the cutting structure, and also beneficially increases compressive stresses in the superhard material table while lowering tensile stresses in the supporting substrate. A number of variations of this approach are described in co-pending U.S. patent application Ser. No. 08/164,481 to Gordon A. Tibbitts, assigned to the assignee of the present invention and incorporated herein by this reference.
It has been proposed to promote heat transfer from a PDC element to the underlying bit structure in U.S. Pat. No. 4,478,297, issued to Robert P. Radtke and assigned on its face to Strata Bit Corporation. The Radtke patent proposed to use a hollow cylindrical stud with a recess extending into about the middle of the stud from the bottom thereof, the recess being filled with a soft, heat-conducting metal to facilitate heat transfer from the PDC at the upper or outer end of the stud.
However, despite the above-referenced developments, it is believed by the inventors that both cutting element stiffness and heat transfer capabilities can be significantly enhanced via the invention described and claimed herein.